The FSA ranks all food providers with a score of zero to five. A zero rating signifies that the establishment “urgently requires improvement”: about 1,400 businesses currently have this score. Between zero and two is considered a failing grade, and three to five is satisfactory.

Filthy conditions and mice: the UK's worst restaurants for hygiene

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The ratings criteria are the same across England, Wales and Northern Ireland and each local authority is responsible for ranking food providers within its boundaries. Scotland uses a different system, awarding a grade of “pass and eat” to premises that exceed legal requirements of hygiene, “pass” to those that are broadly compliant, or “improvement required”.

The analysis revealed huge disparities in hygiene levels around the country, with some local authorities failing large numbers of food establishments and others failing only a handful.

The 10 best areas in the country, ranked by the proportion of food premises that pass inspections, are the Orkney Islands, north Devon, South Ayrshire, Torridge, Harrogate, Cannock Chase, west Dorset, Suffolk coastal, Anglesey and Daventry.

The 10 worst areas in the country are Newham, Edinburgh, the Shetland Islands, Islington, Hyndburn, Harrow, Hertsmere, Midlothian, Luton and Ealing.

The scores also varied across countries and regions. Ten per cent of establishments in Scotland have been awarded a failing grade (improvement required), compared with 6% in England, 5% in Wales and 2% in Northern Ireland.

Businesses are not required to display their food hygiene scores in England and Scotland. In November 2013, Wales made it mandatory for food establishments to publicly display their ratings, which led to a significant improvement in safety. This mandatory display policy will be introduced in Northern Ireland on 7 October.

Sharon Nkansah, a food safety officer, inspects a KFC branch in Newham that received the highest score. Photograph: Martin Godwin for the Guardian

Schools, colleges and universities received the best scores nationwide of any type of food establishment, with less than than 1% given a failing rating. Takeaways and sandwich shops received the worst scores, with 15% judged to be below par.

There are 32 local authorities where more than 25% of takeaways failed to pass food safety inspections. Newham in east London had the highest fail rate for takeaways (50.4%), meaning that customers there were more likely to walk into a takeaway that had failed its inspection than one that had passed. A high proportion of takeaways also failed in Epsom and Ewell (46.2%), Edinburgh (38.2%), Manchester (34%), Birmingham (33.8%) and Islington (32.2%).

Newham also had the worst overall results in London, with 26% of businesses in the borough receiving a score of zero to two.

Only 73 businesses in the country have the worst possible score (a zero overall) and maximum marks (80 out of 80) for concerns about food hygiene, the structure of premises and confidence in management. Of these, 35 are in Birmingham.

While cafe and restaurant chains tended to have very high safety and hygiene ratings, several branches of large chains currently have the worst possible score: Chopstix Noodle Bar in Liverpool, the Poundbakery and Poundcafe at St John’s shopping centre in Liverpool; the Adelphi hotel in Liverpool, which is part of the Britannia Hotels group, and a Costcutter supermarket in Great Hampton Row, Birmingham.

Poundbakery and Poundcafe, Chopstix Noodle Bar and Costcutter said the branches had closed temporarily to deal with hygiene issues, but have since reopened and been deemed to have addressed urgent concerns, and are awaiting inspections to rescore their premises. The Adelphi did not wish to comment.

Businesses are not required to display their food hygiene scores in England and Scotland. Photograph: Martin Godwin for the Guardian

While a zero-rated restaurant will not necessarily be shut down, it will be subject to follow-up inspections and required to address urgent concerns within 28 days of the original visit. The business can then either pay £160 to be reassessed or wait until its next scheduled assessment, usually about six months later, to potentially be given a new score.

Matthew Collins, a principal environmental health officer at Newham council, said a score of zero had to be taken seriously as it indicated a “cumulative failure of management”.

“That would be a failure to keep clean, a failure to maintain accurate temperatures, a failure to disinfect and a failure to have any constructive plan to manage it and/or an infestation of pests. It would be a premises you’d walk into and you’d draw breath,” he said.

“We’re talking about premises we’ve been into where there’s sewage running over the floor, or you stand there and you see something moving out of the corner of your eye, combined with management that have no hold on how to rectify the situation and have obviously let it get into a bad situation with no plan for the future. It’s bad.”

Michael Harding, a food hygiene rating scheme support officer at the FSA, said: “I wouldn’t eat at a zero-rated business. The message from the FSA would be: ‘If it’s got a zero, don’t take the risk.’ Ninety-four per cent of food businesses have a rating of three or better, so there is plenty of choice to eat at a place where they take food hygiene more seriously.”

How to read the ratings

David Pierce, a food safety inspector for 13 years and the principal environmental health officer at Rossendale borough council in Lancashire, said there were a number of reasons for the varying scores across authorities. “You need to look at the types of businesses. A five in a pub, which just does wet sales [drinks], is different to a five in a restaurant; it’s easier to meet the requirements,” he said.

Inspectors can take legal action against businesses that fail to meet standards. In the year ending April 2016, more than 100 successful prosecutions were concluded against food businesses, resulting in penalties of community service, suspended sentences, fines of up to £90,000 and, in one instance, a three-month prison term.

In 2014-15, more than 160,000 food hygiene enforcement actions were issued nationwide, most of which were written warnings (businesses can be issued with multiple actions). Other sanctions included more than 1,000 cases of voluntary closure – in which an establishment agrees to shut while it improves standards – 416 cases of the seizure, detention and surrender of food, and 26 instances of a suspension or revocation of approval of licence.

While a zero-rated restaurant will not necessarily be shut down, it will face follow-up inspections. Photograph: Martin Godwin for the Guardian

Food premises can be inspected as regularly as every six months. For lower-risk businesses, or those with repeatedly high scores, the frequency of inspections can be reduced to once every few years.

Enforcement officers also follow up complaints made by the public. In the year ending March 2015, there were more than 70,000 customer grievances about food establishments. Pierce’s team of inspectors in Rossendale most commonly receive complaints about Indian takeaways, which he described as “unfair”.

“People make these assumptions. Asian food is far more heavily spiced and people don’t necessarily take into account [that] the spicy nature of the food might have an effect on them. The other thing they say is: ‘It was the last thing I ate.’ But they ate it at 2am after six pints and they wonder why they feel sick in the morning,” he said.

However, food safety officers also deal with legitimate complaints, from the relatively minor – one of Pierce’s team is investigating how a sticking plaster found its way on to a customer’s pizza – to more serious incidents, such as E coli in mixed salad leaves.